As the demands on wireless communications continue to increase, the need for ever higher data throughput capacity rises. Various schemes have been proposed to maximise the amount useable bandwidth and to use the available bandwidth as efficiently as possible.
Co-channel interference (CCI) and inter-symbol interference (ISI), which are the two major interference sources in wireless communication, differ from each other in many ways. Two major differences lie in the imbalance of energy and the different characteristics they have in the spatial and temporal domains.
Beamforming is an effective way of excluding interference emanating from a direction other than that of the desired transmitter. This has conventionally been used for narrowband applications, since it is difficult for the beamformer to distinguish between CCI and ISI. However, on the other hand, beamforming can be effective at eliminating strong CCI and ISI interference. The difficulty of beamformers to distinguish between CCI and ISI can reduce the overall efficiency of traditional beamforming applications. By failing to distinguish between these types of interference, any imbalance will adversely affect the performance of the receiver.
Recently, the multiple-input multiple-output (MIMO) system has been investigated. This provides a considerable increase in throughput at relatively small cost in terms of bandwidth. However, the processing overhead for this type of communication technique is significant. In addition, this type of system involves considerable processing overhead in trying to remove inference both from unknown co-channel interference (CCI) from external interferers as well as inter symbol interference (ISI).
When considering unknown interference in a system, adaptive antenna techniques have been proven to be a powerful technique to eliminate the interference efficiently. This kind of interference can be from other systems operating in the same frequency band. However, for a MIMO system, the unknown interference is normally treated as additional noise (coloured-noise) and this will significantly reduce the expected system capacity.
Various techniques have been proposed for dealing with interference. One technique is suggested in a paper by J. W. Liang, et al, entitled “A Two-Stage Hybrid Approach for CCI/ISI Reduction with Space-Time Processing”, IEEE Comm. Letters, Vol. 1, No. 6, November 1997. The system described involves a simple two-stage hybrid approach for CCI and ISI reduction. Two completely separated processes are employed to act on CCI and ISI independently and sequentially. However, the obvious weakness of this arrangement is the great dependency on accurate channel estimation. In fact, with CCI, channel estimation cannot be performed accurately with this technique. Furthermore, this technique is described for use in a single input system.
The papers entitled “A Low Complexity Turbo Adaptive Interference Cancellation Using Antenna Arrays for W-CDMA” by D. Mottier, L. Brunel, IEEE VTC2001-fall and “Iterative Interference Cancellation for High Spectral Efficiency Satellite Communications”, by B. F. Beidas, et al, IEEE Trans. on Comm., Vol. 50, No. 1, January 2002 propose iterative interference cancellation schemes but for use with single-input single-output systems using CDMA. The interference target is multi-user interference, which is specific to CDMA systems.
No single algorithm can employ to operate efficiently on both CCI and ISI. For example, MLSE-type algorithms can effectively equalise ISI but are susceptible to CCI.
The present invention therefore aims to provide a solution to the problems of cancelling ISI and CCI effectively. In particular, the present invention aims to provide an iterative beamforming approach for use in a MIMO system to eliminate unknown interference, which is the interference from other systems which are being operated at the same frequency band.